The brakes on our 5th have never had enough power to lock on pavement. I even increased the wire size to help with voltage drop. That got it to lock on gravel but not on pavement.
We have the lippert "4400" lb axles that are basically a 3500lb axle with a larger diameter tube to minimize deflection under load. This means 3500lb brakes (10" dia) and bearings that are maxed out. Just had a bearing failure after 2 yrs and 5k miles so I'm not a big Lippert fan at the moment.
If your axles are near max rating, they may never lock.
